The present invention relates generally to an engine control system for a turbocharged internal combustion engine, particularly for an automotive gasoline engine. More specifically, the invention relates to a knock-free engine control for an automotive internal combustion engine with a turbocharger for preventing engine knocking and maximizing fuel efficiency and engine performance.
As is well known, a turbocharged engine has a much greater tendency to knock than non-turbocharged engines. This is because the intake air temperature of the turbocharged engine is rather high and the compression in the engine cylinder is higher than in non-turbo charged engines. Therefore, knock control or suppression in the turbocharged engine is more important than in non-turbo charged engines.
Conventionally, various attempts have been made to develop a knock control system responsive to engine knocking to control spark ignition timing and maximum charge pressure in order to avoid harmful engine knocking and increase of exhaust temperature, while simultaneously minimizing engine performance loss and keeping the fuel/performance efficiency at a reasonable level.
One approach has been reported in SAE Technical Paper Series No. 810060 (Feb. 23-27, 1981), by J. E. Rydquist et al. entitled "A Turbocharged Engine with Microprocessor Controlled Boost Pressure". In this report is disclosed an attempt at electronic or microcomputer control of a wastegate setting as well as spark timing when harmful engine knocking occurs. It is found in the report that experiments for knock control were made by controlling ignition timing only, charge pressure boost only and the combination thereof, and the final result is that the combination of ignition timing control and boost control is the most favorable.
As will be appreciated, the best torque range for engine performance varies significantly depending on fuel octane number and/or mechanical octane number. In addition, charge pressure and ignition timing are interrelated with respect to engine output. Therefore, frequent control corrections with reference to various engine operation parameters, such as corrections based on an intake air temperature, transient characteristics and so forth, are required to avoid an excessive increase of the exhaust gas temperature and to limit fluctuation of spark advance angle due to engine knocking in order to maximize engine performance while maintaining suitable fuel efficiency. Thus, knock control in turbocharged engines tends to be quite complicated and difficult.